Stents, in general, may be categorized as permanent, removable, or bioresorbable. Permanent stents are retained in place and incorporated into the lumen wall of a body by promoting ingrowth. Removable stents are removed from the body lumen when the stent is no longer desired. A bioresorbable stent may be composed of, or include, biodegradable material or bioresorbable material which may be broken down by the body and absorbed or passed from the body when it is no longer needed. In treating many bodily vessels, removable stents may be preferred over a permanent stent. For example, many esophageal stenoses procedures require stent removal at a specified date. Further, because it is difficult to predict the exact biodegrading time table of a bioresorbable stent, esophageal technology for instance, in general, has focused on the removable stent prosthesis.
A graft is another type of endoluminal prosthesis which is used to repair and replace body lumina. It is also known to combine a stent and a graft to form a composite stent device. Such composite stent device may provide additional support for weakened sections of a lumen.
The benefits and disadvantages of composite stent devices may be dependent on the extent of the coverings. For instance, bare stents can allow tissue ingrowth in the openings between the struts and therefore may have a low migration rate (undesired longitudinal movement within luminal surfaces). However, bare stents may be very difficult to remove after they have been implanted for a few weeks. In some cases, the ingrowth in the opening between the struts can continue to grow until the patency of the lumen structure is totally obstructed.
Partially covered stents were developed as a means to prevent or slow tissue ingrowth within portions of the body of the stent. However, these configurations also do not totally prevent tissue ingrowth. The tissue ingrowth may occur into openings of cells in the stent wall at the uncovered portions, making the stent still difficult to remove and prone to occlusion by tissue ingrowth.
Fully covered stents are designed to prevent tissue ingrowth along the entire length of the stent and are therefore should be much easier to remove than bare or partially covered stents even after having been implanted for an extended period of time. Covered stents may also slow the growth of tumors and fistula. However, because the covered stents generally do not include any friction inducing structure, their migration rate along the body lumens is higher than the migration rate of bare or partially covered stents. Thus, there is a need for a stent that resists migration and can be easily removed after a given time period.